Add trust_remote_code to GRPOConfig

[muupan]

What does this PR do?

This PR adds trust_remote_code to GRPOConfig and makes GRPOTrainer use it when making the model and its related objects to support custom models.

Fixes #4129

Before submitting

• [x] Did you read the contributor guideline, Pull Request section?
• [x] Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• [x] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

[muupan]

@qgallouedec Can you review this again?